Active material enabled pressure release valves and methods of use

ABSTRACT

An active pressure relief valve adapted for selectively regulating a condition within an interior compartment generally includes a housing fluidly coupling the compartment to an external environment and defining an opening, an actuator including an active material element operable to effect movement of a flap relative to the opening, a load limit protector coupled to and configured to present a secondary output path for the element, when the flap is unable to move, and/or a latching mechanism coupled to, and configured to engage the flap, so as to retain the flap in a modified condition.

BACKGROUND OF THE INVENTION

1. Technical Field

This disclosure generally relates to air pressure relief valves and moreparticularly, to pressure relief valves and methods of regulatingconditions within an interior compartment utilizing active materialactuation.

2. Background Art

It is appreciated that pressure differentials between interior spaces orcompartments and the external environment can cause undesirableconditions. For example, in an automotive setting, it can cause anincrease in “boom” (i.e., low level noise discomfort), required doorclosure force, and especially when sudden, discomfort to the occupantsof the space. As such, pressure relief valves (PRV's) for reducingpressure differential, which may increase upon the closure of a swingpanel (e.g., door, lift flap, rear hatch, etc.) or window, activation ofthe HVAC system or air bag deployment, opening of a window duringmovement of the vehicle, so as to cause a Bernoulli effect. These valvesare generally located within an interior panel that interfaces with thecompartment and the environment (e.g., the structural panel between therear seat and trunk compartment, the structural panel between the floorand the external environment, the structural panel between the dashboardand the engine compartment, and the like). Structurally, conventionalPRV's include at least one conduit fluidly coupling the interior spaceand exterior environment, and a movable flap (e.g., gate) disposed overan opening defined by the conduit. The flap is passively manipulated inresponse to the pressure differential. For example, when air pressurewithin the interior compartment is greater than the external airpressure, the flap opens to compensate for as well as alleviate theincreased pressure; and, when the interior compartment air pressure isless than the external air pressure the movable flap covers the openingto prevent air from entering the interior compartment.

More recently, active PRV's, which utilize a drive mechanism to openand/or close the flap, have been developed to address some of thelimitations of passive PRV's. In these configurations, pressuredifferential is no longer required to actuate the PRV; instead, throughsensory or manual input, it is appreciated that active PRV's can betriggered by and used to address other conditions, such as poor airquality either interior or exterior to the compartment, excessivetemperature, a detect by a sensor, and the operation or status of anassociated system. Active PRV's, however, also present various concernsin the art. For example, prior art active PRV's, including those thatutilize motors, solenoids, and active material actuation (such aspresented by co-owned U.S. Pat. No. 7,204,472 A) to effect the motion ofthe flap, typically require constant power to maintain the valve in themanipulated condition. This invariably results in a drain upon the powersupply. Moreover, with respect to prior art active material based PRV's,the lack of load limit protection resulting in an inability to avoidfailure and the costs associated therewith is also of concern. Forexample, it is appreciated that where the opening of the flap is blockedby a foreign object, the active material element in these PRV's mayoverheat, become damaged, or otherwise fail.

BRIEF SUMMARY

In response to the afore-mentioned concerns, the present inventionrecites an enhanced active material based PRV adapted for use with aninterior compartment, such as that of a vehicle. As a result of thereversible change characteristics exhibited by active materials, theinvention is useful among other things for selectively reducing pressuredifferential, wherein selectivity is based upon a variety ofautonomously or manually determined condition. As a result, theinvention is useful for reducing boom or air bind, as well as,improving/increasing ventilation, on-demand and via remote control.Other advantages of the invention include providing zero power ventposition hold via a latching mechanism. Finally, it is appreciated thatthe novel use of active material elements presented herein reducesweight and/or complexity in comparison to counterpart mechanical,electro-mechanical, hydraulic, or pneumatic based systems; and withrespect to prior-art active material PRV's, increases energy efficiencyand reduces the likelihood of failure and costs associated.

A first aspect of the invention concerns a pressure relief valve adaptedfor use with and for modifying a condition of an interior compartment ofa vehicle. The valve includes a housing defining an opening in fluidcommunication with the compartment and an external environment, a flappivotally connected to the housing, so as to be caused to swing betweenopen and closed conditions, and an actuator drivenly coupled to theflap. The flap is configured to cover at least a portion of the openingin the closed condition and not to obstruct the opening, so as to allowfluid flow between the compartment and environment, in the opencondition. The actuator includes an active material element effective toundergo a reversible change in fundamental property when exposed to anactivation signal. The change is operable to cause the flap to swingbetween the open and closed conditions, so as to achieve a modifiedcondition. The inventive actuator further includes a load limitprotector coupled to and configured to present a secondary output pathfor the element, when the flap is unable to swing between open andclosed conditions, and/or a latching mechanism coupled to, andconfigured to engage the flap, so as to retain the flap in the modifiedcondition even when the change is reversed (e.g., the activation signalis discontinued).

A second aspect of the invention concerns a related method ofselectively modifying a condition of an interior compartment. The methodincludes the steps of fluidly coupling the compartment to an externalenvironment through an opening, so as to allow fluid flow therebetween,securing an active material element relative to the opening, anddetermining a sample value of the condition. Once determined, the samplevalue is compared to a threshold, so as to determine a non-compliantcondition value when the sample value exceeds the threshold. The elementis activated when the non-compliant condition value is determined. Theopening is caused to close or open as a result of activating theelement, and the condition is modified as a result of opening or closingthe opening.

Other aspects and advantages of the present invention, including theemployment of shape memory alloys, shape memory polymers, and otheractive materials for actuating, latching mechanisms for use with, andvarious configurations of active-material based PRV's will be apparentfrom the following detailed description of the preferred embodiment(s)and the accompanying drawing figures.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

A preferred embodiment(s) of the invention is described in detail belowwith reference to the attached drawing figures, wherein:

FIG. 1 is an elevation view of a vehicle including a plurality ofactive-material PRV's coupled to first and second sensors, a controllerand a power supply, in accordance with a preferred embodiment of theinvention;

FIG. 2 is a perspective view of an active PRV, in accordance with apreferred embodiment of the invention;

FIG. 3 is a perspective schematic view of a PRV having a singleswing-door, in accordance with a preferred embodiment of the presentinvention;

FIG. 3 a is a side elevation of the PRV shown in FIG. 3, particularlyillustrating the flap in a closed condition and an active-materialactuator, including a shape memory wire and biasing spring, drivenlycoupled thereto;

FIG. 3 b is a side elevation of the PRV shown in FIGS. 3-3 a, whereinthe flap is in an open condition caused by the activation of the wire;

FIG. 3 c is a side elevation of the PRV shown in FIGS. 3-3 b, whereinthe motion of the flap is obstructed by a foreign object, as the wire isbeing activated;

FIG. 3 d is a side elevation of the PRV shown in FIGS. 3-3 c, whereinthe load limit protector is engaged to provide a secondary output pathfor the element, as a result of the blockage;

FIG. 4 is a side elevation of an active-material based PRV, particularlyillustrating the pivot axis of the flap, and a ratchet based latchingmechanism including a pawl, gear connector, shape memory wire andbiasing spring in enlarged caption view, in accordance with a preferredembodiment of the present invention;

FIG. 4 a is an enlarged caption view of the pivot axis and latchingmechanism shown in FIG. 4, wherein the flap has been swung to an opencondition, such that the pawl is engaged by the gear;

FIG. 4 b is an enlarged caption view of the pivot axis and latchingmechanism shown in FIG. 4, wherein the wire has been activated, so as todisengage the pawl and gear;

FIG. 4 c is an enlarged caption view of the pivot axis and latchingmechanism shown in FIG. 4, wherein the flap has returned to the closedcondition but the wire has not been allowed to cool;

FIG. 5 is a perspective view of an active material based PRV havingmultiple horizontal pivot axes and associated flaps, in accordance witha preferred embodiment of the present invention;

FIG. 6 is a perspective view of an active material based PRV havingmultiple vertical pivot axes and associated flaps, in accordance with apreferred embodiment of the present invention;

FIG. 7 is a side elevation of the PRV shown in FIG. 5, and a topelevation of the PRV shown in FIG. 6, wherein a single active materialactuator engages the flaps, in accordance with a preferred embodiment ofthe present invention;

FIG. 7 a is a side elevation of the PRV shown in FIG. 7, wherein aplurality of separately functioning active material actuators engagesthe flaps to varying degrees, in accordance with a preferred embodimentof the present invention;

FIG. 8 is a perspective view of a single flap PRV having a pivot axismedially located, in accordance with a preferred embodiment of thepresent invention;

FIG. 8 a is a side elevation view of the PRV shown in FIG. 8, whereinthe flap is shown swung to a fully open condition, and in hidden-linetype, back to the fully closed condition;

FIG. 9 is a perspective view taken from the interior of a PRV having acollapsing flap and medially located joint, a shape memory wire in abow-string configuration, and a conduit defining a slot in which thejoint is caused to translate by the bow-string wire, in accordance witha preferred embodiment of the present invention;

FIG. 9 a is a side elevation of the PRV shown in FIG. 9, particularlyillustrating the flap in a closed condition; and

FIG. 9 b is a side elevation of the PRV shown in FIG. 9, particularlyillustrating the flap in a fully open condition, wherein the wire hasbeen activated and the joint translated.

FIG. 10 is a flow diagram of an active-material based PRV process, inaccordance with a preferred embodiment of the present invention.

DETAILED DESCRIPTION

The present invention concerns novel configurations of an activematerial actuated pressure relief valve (PRV) 10 and methods ofmanipulating a cooperatively defined condition, such as pressuredifferential, between an interior compartment or space 12 and anexterior environment 14. In the illustrated and preferred embodimentsdiscussed herein active material based PRV's 10 are employed andutilized with respect to a vehicle 16, such as an automobile, truck,SUV, boat cabin, or airplane; however, it is appreciated that theadvantages and benefits of the present invention may be used in otherapplications or wherever controlling conditions, such as pressuredifferential or air flow between fluidly coupled spaces is desired. Forexample, it is appreciated that the present invention may be applied toresidential and commercial registers.

In the present invention, the use of the active materials provides ameans for selectively opening and closing the pressure relief valves inthe presence or absence of an air pressure differential. As such, theactive pressure relief valves disclosed herein possess addedfunctionality than previously known. For example, as will be disclosedin greater detail herein, the preferred PRV can be programmed to openupon detection of a condition or event. That is to say, the preferredPRV 10 may be programmed (or passively configured) to open when acertain temperature within the interior compartment is exceeded, so thatthe PRV functions as a “smart” vent operable to selectively cool theinterior compartment; and, where the vehicle includes a sensor 18 fordetecting carbon monoxide concentration, for example, the activepressure relief valve could be configured to open when a certainconcentration of carbon monoxide is detected within, thereby allowingexcess carbon monoxide to diffuse from the interior compartment 12.Likewise, as further discussed herein, other sensors 18, such asthermocouples, thermistors, barometers, pre-impact sensors, moisturedetectors, or the like, can be similarly utilized to detect otherconditions and trigger PRV function.

It is appreciated by those of ordinary skill in the art that a pluralityof modes of operation exists for utilizing the active PRV 10 of thepresent invention, including but not limited to a) wherein one or moreactive material actuators cause the valve flap(s) to open, b) whereinone or more active material actuators cause the valve flap(s) to close,c) wherein one or more active material actuators cause the valve flap(s)to both open and close, or d) wherein one or more active materials areused to selectively open and close selectable individual or subgroups ofelements of a multi-element flap. Exemplary embodiments of these casesare illustrated and further described in part (II) below.

I. Active Material Discussion and Function

As used herein the term “active material” shall be afforded its ordinarymeaning as understood by those of ordinary skill in the art, andincludes any material or composite that exhibits a reversible change ina fundamental (e.g., chemical or intrinsic physical) property, whenexposed to an external signal source. Thus, active materials shallinclude those compositions that can exhibit a change in stiffnessproperties, shape and/or dimensions in response to the activationsignal, which can take the type for different active materials, ofelectrical, magnetic, thermal and like fields.

Suitable active materials for use with the present invention include butare not limited to shape memory materials such as shape memory alloys,and shape memory polymers. Shape memory materials generally refer tomaterials or compositions that have the ability to remember theiroriginal at least one attribute such as shape, which can subsequently berecalled by applying an external stimulus. As such, deformation from theoriginal shape is a temporary condition. In this manner, shape memorymaterials can change to the trained shape in response to an activationsignal. Exemplary shape memory materials include the afore-mentionedshape memory alloys (SMA) and shape memory polymers (SMP), as well asshape memory ceramics, electroactive polymers (EAP), ferromagneticSMA's, electrorheological (ER) compositions, magnetorheological (MR)compositions, dielectric elastomers, ionic polymer metal composites(IPMC), piezoelectric polymers, piezoelectric ceramics, variouscombinations of the foregoing materials, and the like.

Shape memory alloys (SMA's) generally refer to a group of metallicmaterials that demonstrate the ability to return to some previouslydefined shape or size when subjected to an appropriate thermal stimulus.Shape memory alloys are capable of undergoing phase transitions in whichtheir yield strength, stiffness, dimension and/or shape are altered as afunction of temperature. The term “yield strength” refers to the stressat which a material exhibits a specified deviation from proportionalityof stress and strain. Generally, in the low temperature, or martensitephase, shape memory alloys can be plastically deformed and upon exposureto some higher temperature will transform to an austenite phase, orparent phase, returning to their shape prior to the deformation.Materials that exhibit this shape memory effect only upon heating arereferred to as having one-way shape memory. Those materials that alsoexhibit shape memory upon re-cooling are referred to as having two-wayshape memory behavior.

Shape memory alloys exist in several different temperature-dependentphases. The most commonly utilized of these phases are the so-calledMartensite and Austenite phases discussed above. In the followingdiscussion, the martensite phase generally refers to the moredeformable, lower temperature phase whereas the austenite phasegenerally refers to the more rigid, higher temperature phase. When theshape memory alloy is in the martensite phase and is heated, it beginsto change into the austenite phase. The temperature at which thisphenomenon starts is often referred to as austenite start temperature(A_(s)). The temperature at which this phenomenon is complete is calledthe austenite finish temperature (A_(f)).

When the shape memory alloy is in the austenite phase and is cooled, itbegins to change into the martensite phase, and the temperature at whichthis phenomenon starts is referred to as the martensite starttemperature (M_(s)). The temperature at which austenite finishestransforming to martensite is called the martensite finish temperature(M_(f)). Generally, the shape memory alloys are softer and more easilydeformable in their martensitic phase and are harder, stiffer, and/ormore rigid in the austenitic phase. In view of the foregoing, a suitableactivation signal for use with shape memory alloys is a thermalactivation signal having a magnitude to cause transformations betweenthe martensite and austenite phases.

Shape memory alloys can exhibit a one-way shape memory effect, anintrinsic two-way effect, or an extrinsic two-way shape memory effectdepending on the alloy composition and processing history. Annealedshape memory alloys typically only exhibit the one-way shape memoryeffect. Sufficient heating subsequent to low-temperature deformation ofthe shape memory material will induce the martensite to austenite typetransition, and the material will recover the original, annealed shape.Hence, one-way shape memory effects are only observed upon heating.Active materials comprising shape memory alloy compositions that exhibitone-way memory effects do not automatically reform, and will likelyrequire an external mechanical force to reform the shape that waspreviously suitable for airflow control.

Intrinsic and extrinsic two-way shape memory materials are characterizedby a shape transition both upon heating from the martensite phase to theaustenite phase, as well as an additional shape transition upon coolingfrom the austenite phase back to the martensite phase. Active materialsthat exhibit an intrinsic shape memory effect are fabricated from ashape memory alloy composition that will cause the active materials toautomatically reform themselves as a result of the above noted phasetransformations. Intrinsic two-way shape memory behavior must be inducedin the shape memory material through processing. Such procedures includeextreme deformation of the material while in the martensite phase,heating-cooling under constraint or load, or surface modification suchas laser annealing, polishing, or shot-peening. Once the material hasbeen trained to exhibit the two-way shape memory effect, the shapechange between the low and high temperature states is generallyreversible and persists through a high number of thermal cycles. Incontrast, active materials that exhibit the extrinsic two-way shapememory effects are composite or multi-component materials that combine ashape memory alloy composition that exhibits a one-way effect withanother element that provides a restoring force to reform the originalshape.

The temperature at which the shape memory alloy remembers its hightemperature form when heated can be adjusted by slight changes in thecomposition of the alloy and through heat treatment. In nickel-titaniumshape memory alloys, for instance, it can be changed from above about100° C. to below about −100° C. The shape recovery process occurs over arange of just a few degrees and the start or finish of thetransformation can be controlled to within a degree or two depending onthe desired application and alloy composition. The mechanical propertiesof the shape memory alloy vary greatly over the temperature rangespanning their transformation, typically providing the system with shapememory effects, superelastic effects, and high damping capacity.

Suitable shape memory alloy materials include, without limitation,nickel-titanium based alloys, indium-titanium based alloys,nickel-aluminum based alloys, nickel-gallium based alloys, copper basedalloys (e.g., copper-zinc alloys, copper-aluminum alloys, copper-gold,and copper-tin alloys), gold-cadmium based alloys, silver-cadmium basedalloys, indium-cadmium based alloys, manganese-copper based alloys,iron-platinum based alloys, iron-platinum based alloys, iron-palladiumbased alloys, and the like. The alloys can be binary, ternary, or anyhigher order so long as the alloy composition exhibits a shape memoryeffect, e.g., change in shape orientation, damping capacity, and thelike.

Thus, for the purposes of this invention, it is appreciated that SMA'sexhibit a modulus increase of 2.5 times and a dimensional change of upto 8% (depending on the amount of pre-strain) when heated above theirMartensite to Austenite phase transition temperature. It is appreciatedthat thermally induced SMA phase changes are one-way so that a biasingforce return mechanism (such as a spring) would be required to returnthe SMA to its starting configuration once the applied field is removed.Joule heating can be used to make the entire system electronicallycontrollable. Stress induced phase changes in SMA are, however, two wayby nature. Application of sufficient stress when an SMA is in itsAustenitic phase will cause it to change to its lower modulusMartensitic phase in which it can exhibit up to 8% of “superelastic”deformation. Removal of the applied stress will cause the SMA to switchback to its Austenitic phase in so doing recovering its starting shapeand higher modulus.

Ferromagnetic SMA's (FSMA's), which are a sub-class of SMAs, may also beused in the present invention. These materials behave like conventionalSMA materials that have a stress or thermally induced phasetransformation between martensite and austenite. Additionally FSMA's areferromagnetic and have strong magnetocrystalline anisotropy, whichpermit an external magnetic field to influence the orientation/fractionof field aligned martensitic variants. When the magnetic field isremoved, the material may exhibit complete two-way, partial two-way orone-way shape memory. For partial or one-way shape memory, an externalstimulus, temperature, magnetic field or stress may permit the materialto return to its starting state. Perfect two-way shape memory may beused for proportional control with continuous power supplied. One-wayshape memory is most useful for rail filling applications. Externalmagnetic fields are generally produced via soft-magnetic coreelectromagnets in automotive applications, though a pair of Helmholtzcoils may also be used for fast response.

Shape memory polymers (SMP's) generally refer to a group of polymericmaterials that demonstrate the ability to return to a previously definedshape when subjected to an appropriate thermal stimulus. Shape memorypolymers are capable of undergoing phase transitions in which theirshape is altered as a function of temperature. Generally, SMP's have twomain segments, a hard segment and a soft segment. The previously definedor permanent shape can be set by melting or processing the polymer at atemperature higher than the highest thermal transition followed bycooling below that thermal transition temperature. The highest thermaltransition is usually the glass transition temperature (T_(g)) ormelting point of the hard segment. A temporary shape can be set byheating the material to a temperature higher than the T_(g) or thetransition temperature of the soft segment, but lower than the T_(g) ormelting point of the hard segment. The temporary shape is set whileprocessing the material at the transition temperature of the softsegment followed by cooling to fix the shape. The material can bereverted back to the permanent shape by heating the material above thetransition temperature of the soft segment.

For example, the permanent shape of the polymeric material may be a wirepresenting a substantially straightened shape and defining a firstlength, while the temporary shape may be a similar wire defining asecond length less than the first. In another embodiment, the materialmay present a spring having a first modulus of elasticity when activatedand second modulus when deactivated.

The temperature needed for permanent shape recovery can be set at anytemperature between about −63° C. and about 120° C. or above.Engineering the composition and structure of the polymer itself canallow for the choice of a particular temperature for a desiredapplication. A preferred temperature for shape recovery is greater thanor equal to about −30° C., more preferably greater than or equal toabout 0° C., and most preferably a temperature greater than or equal toabout 50° C. Also, a preferred temperature for shape recovery is lessthan or equal to about 120° C., and most preferably less than or equalto about 120° C. and greater than or equal to about 80° C.

Suitable shape memory polymers include thermoplastics, thermosets,interpenetrating networks, semi-interpenetrating networks, or mixednetworks. The polymers can be a single polymer or a blend of polymers.The polymers can be linear or branched thermoplastic elastomers withside chains or dendritic structural elements. Suitable polymercomponents to form a shape memory polymer include, but are not limitedto, polyphosphazenes, poly(vinyl alcohols), polyamides, polyesteramides, poly(amino acid)s, polyanhydrides, polycarbonates,polyacrylates, polyalkylenes, polyacrylamides, polyalkylene glycols,polyalkylene oxides, polyalkylene terephthalates, polyortho esters,polyvinyl ethers, polyvinyl esters, polyvinyl halides, polyesters,polylactides, polyglycolides, polysiloxanes, polyurethanes, polyethers,polyether amides, polyether esters, and copolymers thereof. Examples ofsuitable polyacrylates include poly(methyl methacrylate), poly(ethylmethacrylate), ply(butyl methacrylate), poly(isobutyl methacrylate),poly(hexyl methacrylate), poly(isodecyl methacrylate), poly(laurylmethacrylate), poly(phenyl methacrylate), poly(methyl acrylate),poly(isopropyl acrylate), poly(isobutyl acrylate) and poly(octadecylacrylate). Examples of other suitable polymers include polystyrene,polypropylene, polyvinyl phenol, polyvinylpyrrolidone, chlorinatedpolybutylene, poly(octadecyl vinyl ether) ethylene vinyl acetate,polyethylene, poly(ethylene oxide)-poly(ethylene terephthalate),polyethylene/nylon (graft copolymer), polycaprolactones-polyamide (blockcopolymer), poly(caprolactone) dimethacrylate-n-butyl acrylate,poly(norbornyl-polyhedral oligomeric silsequioxane), polyvinylchloride,urethane/butadiene copolymers, polyurethane block copolymers,styrene-butadiene-styrene block copolymers, and the like.

Thus, for the purposes of this invention, it is appreciated that SMP'sexhibit a dramatic drop in modulus when heated above the glasstransition temperature of their constituent that has a lower glasstransition temperature. If loading/deformation is maintained while thetemperature is dropped, the deformed shape will be set in the SMP untilit is reheated while under no load under which condition it will returnto its as-molded shape. While SMP's could be used variously in block,sheet, slab, lattice, truss, fiber or foam forms, they requirecontinuous power to remain in their lower modulus state. Thus, they aresuited for reversible shape setting of the insert 10.

Suitable piezoelectric materials include, but are not intended to belimited to, inorganic compounds, organic compounds, and metals. Withregard to organic materials, all of the polymeric materials withnon-centrosymmetric structure and large dipole moment group(s) on themain chain or on the side-chain, or on both chains within the molecules,can be used as suitable candidates for the piezoelectric film. Exemplarypolymers include, for example, but are not limited to, poly(sodium4-styrenesulfonate), poly (poly(vinylamine)backbone azo chromophore),and their derivatives; polyfluorocarbons, includingpolyvinylidenefluoride, its co-polymer vinylidene fluoride (“VDF”),co-trifluoroethylene, and their derivatives; polychlorocarbons,including poly(vinyl chloride), polyvinylidene chloride, and theirderivatives; polyacrylonitriles, and their derivatives; polycarboxylicacids, including poly(methacrylic acid), and their derivatives;polyureas, and their derivatives; polyurethanes, and their derivatives;bio-molecules such as poly-L-lactic acids and their derivatives, andcell membrane proteins, as well as phosphate bio-molecules such asphosphodilipids; polyanilines and their derivatives, and all of thederivatives of tetramines; polyamides including aromatic polyamides andpolyimides, including Kapton and polyetherimide, and their derivatives;all of the membrane polymers; poly(N-vinyl pyrrolidone) (PVP)homopolymer, and its derivatives, and random PVP-co-vinyl acetatecopolymers; and all of the aromatic polymers with dipole moment groupsin the main-chain or side-chains, or in both the main-chain and theside-chains, and mixtures thereof.

Piezoelectric materials can also comprise metals selected from the groupconsisting of lead, antimony, manganese, tantalum, zirconium, niobium,lanthanum, platinum, palladium, nickel, tungsten, aluminum, strontium,titanium, barium, calcium, chromium, silver, iron, silicon, copper,alloys comprising at least one of the foregoing metals, and oxidescomprising at least one of the foregoing metals. Suitable metal oxidesinclude SiO.sub.2, Al.sub.2O.sub.3, ZrO.sub.2, TiO.sub.2, SrTiO.sub.3,PbTiO.sub.3, BaTiO.sub.3, FeO.sub.3, Fe.sub.3O.sub.4, ZnO, and mixturesthereof and Group VIA and JIB compounds, such as CdSe, CdS, GaAs,AgCaSe.sub.2, ZnSe, GaP, InP, ZnS, and mixtures thereof. Preferably, thepiezoelectric material is selected from the group consisting ofpolyvinylidene fluoride, lead zirconate titanate, and barium titanate,and mixtures thereof.

Suitable magnetorheological fluid materials include, but are notintended to be limited to, ferromagnetic or paramagnetic particlesdispersed in a carrier fluid. Suitable particles include iron; ironalloys, such as those including aluminum, silicon, cobalt, nickel,vanadium, molybdenum, chromium, tungsten, manganese and/or copper; ironoxides, including Fe.sub.2O.sub.3 and Fe.sub.3O.sub.4; iron nitride;iron carbide; carbonyl iron; nickel and alloys of nickel; cobalt andalloys of cobalt; chromium dioxide; stainless steel; silicon steel; andthe like. Examples of suitable particles include straight iron powders,reduced iron powders, iron oxide powder/straight iron powder mixturesand iron oxide powder/reduced iron powder mixtures. A preferredmagnetic-responsive particulate is carbonyl iron, preferably, reducedcarbonyl iron.

The particle size should be selected so that the particles exhibitmulti-domain characteristics when subjected to a magnetic field.Diameter sizes for the particles can be less than or equal to about1,000 micrometers, with less than or equal to about 500 micrometerspreferred, and less than or equal to about 100 micrometers morepreferred. Also preferred is a particle diameter of greater than orequal to about 0.1 micrometer, with greater than or equal to about 0.5more preferred, and greater than or equal to about 10 micrometersespecially preferred. The particles are preferably present in an amountbetween about 5.0 to about 50 percent by volume of the total MR fluidcomposition.

Suitable carrier fluids include organic liquids, especially non-polarorganic liquids. Examples include, but are not limited to, siliconeoils; mineral oils; paraffin oils; silicone copolymers; white oils;hydraulic oils; transformer oils; halogenated organic liquids, such aschlorinated hydrocarbons, halogenated paraffins, perfluorinatedpolyethers and fluorinated hydrocarbons; diesters; polyoxyalkylenes;fluorinated silicones; cyanoalkyl siloxanes; glycols; synthetichydrocarbon oils, including both unsaturated and saturated; andcombinations comprising at least one of the foregoing fluids.

The viscosity of the carrier component can be less than or equal toabout 100,000 centipoise, with less than or equal to about 10,000centipoise preferred, and less than or equal to about 1,000 centipoisemore preferred. Also preferred is a viscosity of greater than or equalto about 1 centipoise, with greater than or equal to about 250centipoise preferred, and greater than or equal to about 500 centipoiseespecially preferred.

Aqueous carrier fluids may also be used, especially those comprisinghydrophilic mineral clays such as bentonite or hectorite. The aqueouscarrier fluid may comprise water or water comprising a small amount ofpolar, water-miscible organic solvents such as methanol, ethanol,propanol, dimethyl sulfoxide, dimethyl formamide, ethylene carbonate,propylene carbonate, acetone, tetrahydrofuran, diethyl ether, ethyleneglycol, propylene glycol, and the like. The amount of polar organicsolvents is less than or equal to about 5.0% by volume of the total MRfluid, and preferably less than or equal to about 3.0%. Also, the amountof polar organic solvents is preferably greater than or equal to about0.1%, and more preferably greater than or equal to about 1.0% by volumeof the total MR fluid. The pH of the aqueous carrier fluid is preferablyless than or equal to about 13, and preferably less than or equal toabout 9.0. Also, the pH of the aqueous carrier fluid is greater than orequal to about 5.0, and preferably greater than or equal to about 8.0.

Natural or synthetic bentonite or hectorite may be used. The amount ofbentonite or hectorite in the MR fluid is less than or equal to about 10percent by weight of the total MR fluid, preferably less than or equalto about 8.0 percent by weight, and more preferably less than or equalto about 6.0 percent by weight. Preferably, the bentonite or hectoriteis present in greater than or equal to about 0.1 percent by weight, morepreferably greater than or equal to about 1.0 percent by weight, andespecially preferred greater than or equal to about 2.0 percent byweight of the total MR fluid.

Optional components in the MR fluid include clays, organoclays,carboxylate soaps, dispersants, corrosion inhibitors, lubricants,extreme pressure anti-wear additives, antioxidants, thixotropic agentsand conventional suspension agents. Carboxylate soaps include ferrousoleate, ferrous naphthenate, ferrous stearate, aluminum di- andtri-stearate, lithium stearate, calcium stearate, zinc stearate andsodium stearate, and surfactants such as sulfonates, phosphate esters,stearic acid, glycerol monooleate, sorbitan sesquioleate, laurates,fatty acids, fatty alcohols, fluoroaliphatic polymeric esters, andtitanate, aluminate and zirconate coupling agents and the like.Polyalkylene diols, such as polyethylene glycol, and partiallyesterified polyols can also be included.

Suitable MR elastomer materials include, but are not intended to belimited to, an elastic polymer matrix comprising a suspension offerromagnetic or paramagnetic particles, wherein the particles aredescribed above. Suitable polymer matrices include, but are not limitedto, poly-alpha-olefins, natural rubber, silicone, polybutadiene,polyethylene, polyisoprene, and the like.

Electroactive polymers include those polymeric materials that exhibitpiezoelectric, pyroelectric, or electrostrictive properties in responseto electrical or mechanical fields. An example of anelectrostrictive-grafted elastomer with a piezoelectric poly(vinylidenefluoride-trifluoro-ethylene) copolymer. This combination has the abilityto produce a varied amount of ferroelectric-electrostrictive, molecularcomposite systems. These may be operated as a piezoelectric sensor oreven an electrostrictive actuator.

Materials suitable for use as an electroactive polymer may include anysubstantially insulating polymer or rubber (or combination thereof) thatdeforms in response to an electrostatic force or whose deformationresults in a change in electric field. Exemplary materials suitable foruse as a pre-strained polymer include silicone elastomers, acrylicelastomers, polyurethanes, thermoplastic elastomers, copolymerscomprising PVDF, pressure-sensitive adhesives, fluoroelastomers,polymers comprising silicone and acrylic moieties, and the like.Polymers comprising silicone and acrylic moieties may include copolymerscomprising silicone and acrylic moieties, polymer blends comprising asilicone elastomer and an acrylic elastomer, for example.

Materials used as an electroactive polymer may be selected based on oneor more material properties such as a high electrical breakdownstrength, a low modulus of elasticity—(for large or small deformations),a high dielectric constant, and the like. In one embodiment, the polymeris selected such that is has an elastic modulus at most about 100 MPa.In another embodiment, the polymer is selected such that is has amaximum actuation pressure between about 0.05 MPa and about 10 MPa, andpreferably between about 0.3 MPa and about 3 MPa. In another embodiment,the polymer is selected such that is has a dielectric constant betweenabout 2 and about 20, and preferably between about 2.5 and about 12. Thepresent disclosure is not intended to be limited to these ranges.Ideally, materials with a higher dielectric constant than the rangesgiven above would be desirable if the materials had both a highdielectric constant and a high dielectric strength. In many cases,electroactive polymers may be fabricated and implemented as thin films.Thicknesses suitable for these thin films may be below 50 micrometers.

As electroactive polymers may deflect at high strains, electrodesattached to the polymers should also deflect without compromisingmechanical or electrical performance. Generally, electrodes suitable foruse may be of any shape and material provided that they are able tosupply a suitable voltage to, or receive a suitable voltage from, anelectroactive polymer. The voltage may be either constant or varyingover time. In one embodiment, the electrodes adhere to a surface of thepolymer. Electrodes adhering to the polymer are preferably compliant andconform to the changing shape of the polymer. Correspondingly, thepresent disclosure may include compliant electrodes that conform to theshape of an electroactive polymer to which they are attached. Theelectrodes may be only applied to a portion of an electroactive polymerand define an active area according to their geometry. Various types ofelectrodes suitable for use with the present disclosure includestructured electrodes comprising metal traces and charge distributionlayers, textured electrodes comprising varying out of plane dimensions,conductive greases such as carbon greases or silver greases, colloidalsuspensions, high aspect ratio conductive materials such as carbonfibrils and carbon nanotubes, and mixtures of ionically conductivematerials.

Materials used for electrodes of the present disclosure may vary.Suitable materials used in an electrode may include graphite, carbonblack, colloidal suspensions, thin metals including silver and gold,silver filled and carbon filled gels and polymers, and ionically orelectronically conductive polymers. It is understood that certainelectrode materials may work well with particular polymers and may notwork as well for others. By way of example, carbon fibrils work wellwith acrylic elastomer polymers while not as well with siliconepolymers.

II. Exemplary Active-Material PRV's, Methods, and Applications

Turning to the structural configuration of the invention, there is shownvarious embodiments of an active pressure relief valve 10 that utilizesactive material actuation in FIGS. 1-10. In general, the PRV 10 includesa housing (or conduit) 20 defining an opening 22 (FIG. 2); a hinge 24rotate-ably mounted to the housing 20; a rigid flap 26 in pivotablecommunication with the hinge 24; and a control connector 28 in operativecommunication with an active material based actuator 30. The housing 20may be fixedly mounted to the vehicle 16, as shown in FIG. 1, or otherstructure. Though singularly described and illustrated, it isappreciated that a plurality of PRV's may be utilized, and separatelycontrolled with respect to the interior compartment (FIG. 1).

The opening 22 is in fluid communication with the interior compartment12 and the external environment 14. In this manner, selective openingand closing of the flap 26 can be used to regulate fluid flow betweenthe interior compartment 12 and the external environment 14. To furtherprevent fluid flow, the PRV 10 may further comprise an elastic seal (notshown) disposed about the opening 22, and intermediate the flap 26 andhousing 20, so as to be compressed thereby. Also as further describedbelow, multiple flaps operating in unison or individually, or otheradjustment means, can be employed to variably control fluidcommunication.

A power supply 32 is in operative communication with the actuator 30 andoperable to provide a suitable activation signal (FIG. 1). The powersupply 32 may be automatically demanded via remote control, andregulated by a PWM, regulator, or power resistor in-series. For example,in the case of actuators comprising thermally activated shape memorymaterial, a current can be supplied by the power supply 32 to effectJoule heating, when demanded by a vehicle occupant (not shown).Alternatively, the power supply 32 may come from an ambient energy orcondition source, such as radiation from the Sun, so that the PRV 10 ispassively activated.

Referring to FIGS. 3-3 d, there is shown a PRV 10 having a rigid flap 26pivotally mounted along the top of the housing 20, so as to define apivot axis, p. Connector 28 presents a swing arm that defines a pivotpoint concentrically aligned with the pivot axis of the flap 26, a longarm 34, and a short arm 36. The long arm 34 coextends with the flap 26and defines a first arm length equal to the longitudinal dimension ofthe flap 26. The short arm 36 presents a second arm length preferablyless than half, more preferably less than one-quarter, and mostpreferably less than one-eighth of the first arm length.

An active material element 38 of defined length is attached to the shortarm 36. In FIGS. 3-3 d, the element 38 consists of a shape memory wirethat is attached at one end to the short arm 36 and to the housing 20 orvehicle 16 at the other end. Upon activation by the power supply 32, thelength of the wire 38 decreases causing the rigid flap 26 to pivot aboutits axis. Upon discontinuing the activation signal, the wire 38 returnsto its original dimension or undergoes a plastic deformation, dependingon the active material employed to effect closure of the flap 26. Asshown in FIG. 3 a-b, once activated the wire 38 is caused to swing as aresult of rotation by the short arm 36; to prevent stress accumulationand/or buckling at the other end, it is desirous to pivotally connectthe wire 38 and housing 20.

It is appreciated that the preferred wire 38 presents stress and strainvalues of 170 MPa and 2.5%, respectively, so as to result in a sealingforce of 2N, when activated, and that between 2.5 to 12 V, and 2 amps ofcurrent are required to actuate the PRV 10.

More preferably, as shown in FIG. 3 b an extension spring 40 is alsoattached to the short arm 36 and configured to produce the plasticdeformation and/or effect closure of the flap 26, so that the flap 26 ismaintained against the housing 20 in the power-off state (alternatively,a torsion spring may be disposed about the pivot axis and configured toact in the same manner). The bias spring 40 is able to stretch the shapememory wire 38, so as to cause stress-induced transformation to amartensite phase, in addition to that caused by cooling.

With respect to case b) above, it is appreciated that theafore-mentioned configurations could be reversed, wherein a compressionspring (not shown) works to drive the flap 26 open and the wire 38 worksto selectively close the valve. With respect to case c), it is alsoappreciated that two antagonistic active material actuators 30, onewhich would open, and another which would close the flap 26 (oralternatively, a single actuator having a two-way effect), could beutilized.

An aspect of the invention concerns the inclusion of a load limitprotector 52 to provide strain/stress relief capability, and therebyincrease the life of the element 38. In this regard, it is appreciatedthat when an active material undergoes transformation, but is preventedfrom undergoing the resultant physical change (e.g. heating a stretchedSMA wire above its transformation temperature but not allowing the wireto revert to its unstressed state), detrimental affects to the materialsperformance and/or longevity can occur. In the present invention, forexample, it is foreseeable that the flap 26 could by constrained frommoving when actuated, either by a foreign object 54 (FIGS. 3 c-d) oranother form of impediment (e.g., a deformed out body panel blockingmotion or ice/mud build up on the valve flap). As such, to preventdamage to the actuator element 38, a secondary output path to thetriggered motion is preferably included, which allows the element 38 torespond to the activation signal while the condition of flap 26 remainsunchanged.

For example, the wire 38 may be further connected to an extension spring56 placed in series therewith, opposite the connector 28 (FIGS. 3 a-d).The spring 56 is stretched to a point where its applied preloadcorresponds to the load level where it is appreciated that the actuator30 would begin to experience excessive force if blocked. As a result,activation of the actuator 30 will first apply a force trying to openthe flap 26, but if the force level exceeds the preload in the spring(e.g., the flap 26 is blocked), the wire 38 will instead further stretchthe spring, thereby providing an output path for the wire strain, andpreserving the integrity of the active PRV 10.

More preferably, and as shown in FIGS. 3-3 d, the protector 52 mayfurther include a lever 58 intermediate the element 38 and spring 56.The lever 58 defines first and second arms 60,62 and a pivot axis. Theelement 38 is attached to one of the arms, so as to be spaced from theaxis a first distance. The spring 56 is attached to the other arm andspaced from the axis a second distance greater than the first, so as toprovide mechanical advantage.

If overload protection fails, it is also appreciated that a mechanicallink, such as a cable (not shown) attached to the interior of a PRV 10that could be accessed and pulled to close the PRV 10 should it fail ina partially to fully open condition, especially in the case ofembodiments with a bias spring opening and active material based closingmechanisms.

The inventive PRV 10 preferably includes a latching mechanism, such asthe type shown in FIGS. 4-4 c. In this configuration the mechanismincludes a pawl 44 resistively pivotable about an axis between engagedand disengaged positions, and the connector 28 presents a rotatable gear46 defining at least one tooth configured to catch the pawl 44 when inthe engaged position. More preferably, the gear 46 has a plurality ofteeth so as to be able to engage the pawl 44 at a plurality ofincremental positions resulting in variable degrees of opening (e.g.,between 17° and 52°) for the flap 26. In the illustrated embodiment, abiasing latch spring 48 attached to the pawl 44 is configured to causethe mechanism 42 to engage the flap 26, and a second active materialelement (e.g., shape memory wire) 50 presenting an activation forcegreater than the spring modulus of the spring 48 is oppositely attachedto the pawl 44. As shown in FIG. 4 b, the second element 50 is operableto cause the mechanism 42 to disengage and release the flap 26, whenactivated. Alternatively, it is appreciated that the roles of theelement 50 and spring 48 may be reversed by switching their connectionpoints relative to the pawl 44; that is to say the second element 50could be configured to cause the mechanism 42 to engage the flap 26 whenactivated, and the biasing spring 48 presents a spring modulus less thanthe activation force, so as to be configured to cause the mechanism 42to disengage and release the flap 26, only when the second element 50 isdeactivated.

Based on these configurations, actuation times of less than fiveseconds, an approximate lifetime of 100,000 actuations, and a workingenvironment between −40 to 90° C. have been observed.

In other embodiments, FIGS. 5-7 a depict PRV's 10 having a plurality ofhorizontal or vertically oriented flaps 26 that are each pivotallyconnected to the housing 20 so as to define an equal plurality of pivotaxes. A single actuator 30 may be connected to each of the flaps 26 andconfigure to effect congruent motion, as previously described and shownin FIG. 7, or more preferably, a separate active material actuator 30may control movement of an associative flap 26 as shown in FIG. 7 a.Again, the flaps 26 are preferably biased so as to sealingly close aportion of the opening, when the associative element 38 is deactivated.It is also within the ambit of the invention to variously employrotating, folding, sliding or iris-type flaps 26 consistent with theinvention.

In another embodiment, the flap 26 may present a medial pivot axis, soas to be able to rotate about its longitudinal mid-line, as shown inFIGS. 8 and 8 a. In this configuration it is appreciated that half ofthe flap extends within the housing 20 when in the open conditionresulting in less protrusion into the compartment 12. It is contemplatedthat other active material based actuators, such as a torque tubecoupled with an antagonistically biased torsion spring, may beimplemented in this configuration.

Finally, in yet another preferred embodiment, the housing 20 may definea medial slot 66 and include first and second flap engagingcross-members 68,70 opposite the slot 66 (FIGS. 9-9 b). The flap 26, inthis configuration, is collapsible about a medial pivot joint formed bya hinge 24. The joint is disposed within the slot 66. The members 68,70cause the flap 26 to collapse by folding, when the joint is caused totranslate towards the interior of the housing 20. In this configuration,a shape memory wire 38 is preferably coupled to the joint and defines abow-string configuration (FIGS. 9 a,b). As a result of trigonometricrelationship, when the wire 38 is activated, the bow-stringconfiguration causes the joint to translate a distance greater than thewire displacement (i.e., change in length). A return spring 72presenting a spring modulus less than the activation force of the wire38 is disposed within the slot 66 and configured to maintain engagementwith, so as to bias the joint outward. Finally, a torsion spring 74 iscoaxially aligned with the hinge 24, attached to the flap 26, andconfigured such that as the flap 26 is caused to collapse, i.e., by thetranslation of the joint inward, the spring 74 is caused to storepotential energy. When the wire 38 is deactivated and the joint iscaused to travel outward, the torsion spring 74 releases its energy, soas to further cause the flap 26 to returns to and be sealed in theclosed condition.

In operation, selective opening of the rigid flap 26 can be effected bya controller 64 communicatively coupled to the power supply 30, andsensor(s) 18 and/or input device. The controller 64 can be preprogrammedto have the power supply 32 deliver the activation signal usingalgorithms based on sensor input, as previously described and exemplaryrepresented in FIG. 10. For example, air pressure can be monitored witha sensor to indicate when a window is opened during vehicle movement,when an air bag is actuated, or upon door closing, or the like.Alternatively, other sensor inputs such a temperature indicating thatthe interior temperature of the vehicle has exceeded a pre-settemperature threshold, and the like can be employed. Still, other sensorinputs can include a gas sensor such as may be desired for detectingcarbon monoxide concentration within the interior compartment.

In other embodiments, it is appreciated that the operation (eitheropening or closing) of the PRV 10 could be triggered by the actuation ofan HVAC system operable to treat the interior compartment 12, or byreceiving telematics information cooperatively determined from a GPS orother positioning system and a map database (FIG. 10); both of whichbeing also communicatively coupled to the controller 64, and suitablefor implementation in an automotive setting, for example.

Ranges disclosed herein are inclusive and combinable (e.g., ranges of“up to about 25 wt %, or, more specifically, about 5 wt % to about 20 wt%”, is inclusive of the endpoints and all intermediate values of theranges of “about 5 wt % to about 25 wt %,” etc.). “Combination” isinclusive of blends, mixtures, alloys, reaction products, and the like.Furthermore, the terms “first,” “second,” and the like, herein do notdenote any order, quantity, or importance, but rather are used todistinguish one element from another, and the terms “a” and “an” hereindo not denote a limitation of quantity, but rather denote the presenceof at least one of the referenced item. The modifier “about” used inconnection with a quantity is inclusive of the state value and has themeaning dictated by context, (e.g., includes the degree of errorassociated with measurement of the particular quantity). The suffix“(s)” as used herein is intended to include both the singular and theplural of the term that it modifies, thereby including one or more ofthat term (e.g., the colorant(s) includes one or more colorants).Reference throughout the specification to “one embodiment”, “anotherembodiment”, “an embodiment”, and so forth, means that a particularelement (e.g., feature, structure, and/or characteristic) described inconnection with the embodiment is included in at least one embodimentdescribed herein, and may or may not be present in other embodiments. Inaddition, it is to be understood that the described elements may becombined in any suitable manner in the various embodiments.

Suitable algorithms, processing capability, and sensor inputs are wellwithin the skill of those in the art in view of this disclosure. Thisinvention has been described with reference to exemplary embodiments; itwill be understood by those skilled in the art that various changes maybe made and equivalents may be substituted for elements thereof withoutdeparting from the scope of the invention. In addition, manymodifications may be made to adapt a particular situation or material tothe teachings of the invention without departing from the essentialscope thereof. Therefore, it is intended that the invention not belimited to a particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

1. A pressure relief valve adapted for use with and for modifying acondition of an interior compartment, said valve comprising: a housingdefining an opening in fluid communication with the compartment and anexternal environment; at least one flap pivotally connected to thehousing, so as to be caused to swing between open and closed conditions,and configured to cover at least a portion of the opening in the closedcondition and not to obstruct the opening, so as to allow fluid flowbetween the compartment and environment, in the open condition; and atleast one actuator drivenly coupled to the flap, and including an activematerial element effective to undergo a reversible change in fundamentalproperty when exposed to an activation signal, wherein the change isoperable to cause the flap to swing between the open and closedconditions, so as to achieve a modified condition, said actuator furthercomprising a latching mechanism coupled to, and configured to engage theflap, so as to retain the flap in the modified condition when the changeis reversed.
 2. The valve as claimed in claim 1, wherein the activematerial is selected from the group consisting essentially of shapememory alloys, ferromagnetic shape memory alloys, shape memory polymers,piezoelectric materials, electroactive polymers, magnetorheologicalfluids and elastomers, electroreheological fluids, and composites of thesame.
 3. The valve as claimed in claim 1, wherein the latching mechanismincludes biasing spring configured to cause the mechanism to engage theflap, and a second active material element presenting an activationforce greater than the spring modulus of the spring, so as to beoperable to cause the mechanism to disengage and release the flap, whenthe mechanism is engaged with the flap and the second element isactivated.
 4. The valve as claimed in claim 1, wherein the latchingmechanism includes a second active material element presenting anactivation force and configured to cause the mechanism to engage theflap when activated, and a biasing spring presenting a spring modulusless than the activation force, so as to be configured to cause themechanism to disengage and release the flap, when the mechanism isengaged with the flap and the second element is deactivated.
 5. Thevalve as claimed in claim 1, wherein the mechanism includes a pawlresistively pivotable between engaged and disengaged positions, and arotatable gear defining at least one tooth configured to catch the pawlwhen in the engaged position.
 6. The valve as claimed in claim 1,further comprising a load limit protector coupled to and configured topresent a secondary output path for the element, when the flap is unableto swing between open and closed conditions.
 7. The valve as claimed inclaim 1, wherein the flap presents a medial pivot axis, so that half ofthe flap extends within the housing when in the open condition.
 8. Thevalve as claimed in claim 1, wherein a plurality of flaps are pivotableconnected to the housing so as to define an equal plurality of pivotaxes, and each of the flaps are drivenly coupled to a separatelyoperable actuator.
 9. The valve as claimed in claim 1, wherein thehousing defines a medial slot and includes first and second flapengaging members opposite the slot, the flap is collapsible about amedial pivot joint disposed within the slot, and caused to collapse bythe members when the joint is caused to translate within the slot andtowards the interior of the housing.
 10. The valve as claimed in claim1, wherein the actuator includes a shape memory wire presenting a wiredisplacement when activated or deactivated, and the wire is coupled tothe flap and defines a bow-string configuration, such that the flap iscaused to translate a distance greater than the displacement.
 11. Apressure relief valve adapted for use with and for modifying a conditionof an interior compartment of a vehicle, said valve comprising: ahousing defining an opening in fluid communication with the compartmentand an external environment; a flap pivotally connected to the housing,so as to be caused to swing between open and closed conditions, andconfigured to cover at least a portion of the opening in the closedcondition and not to obstruct the opening, so as to allow fluid flowbetween the compartment and environment, in the open condition; and anactuator drivenly coupled to the flap, and including an active materialelement effective to undergo a reversible change in fundamental propertywhen exposed to an activation signal, wherein the change is operable tocause the flap to swing between the open and closed conditions, so as toachieve a modified condition, said actuator further comprising a loadlimit protector coupled to and configured to present a secondary outputpath for the element, when the flap is unable to swing between open andclosed conditions.
 12. The valve as claimed in claim 11, wherein theprotector includes a stretched spring connected in series to theelement.
 13. The valve as claimed in claim 12, wherein the protectorfurther includes a lever intermediate the element and spring, the leverdefines first and second arms and a pivot axis, the element is attachedto one of said arms and spaced from the axis a first distance, and thespring is attached to the other of said arms and spaced from the axis asecond distance greater than the first.
 14. A method of selectivelymodifying a condition of an interior compartment, said method comprisingthe steps of: a. fluidly coupling the compartment to an externalenvironment through an opening, so as to allow fluid flow therebetween;b. securing an active material element relative to the opening; c.determining a sample value of the condition; d. comparing the samplevalue to a threshold, and determining a non-compliant condition value,when the sample value exceeds the threshold; e. activating the elementwhen the non-compliant condition value is determined; f. causing theopening to be closed or opened as a result of activating the element;and g. modifying the condition as a result of opening or closing theopening.
 15. The method as claimed in claim 14, wherein the condition iscooperatively defined by the compartment and environment, and selectedfrom the group consisting essentially of a pressure differential, atemperature differential, a gaseous concentration differential, andcombinations thereof.
 16. The method as claimed in claim 14, whereinstep c) further includes the steps of securing at least one sensorrelative to the compartment and detecting the value using the sensor.17. The method as claimed in claim 16, wherein the sensor is athermocouple, barometer, a pre-impact sensor, moisture detector, orcarbon monoxide sensor.
 18. The method as claimed in claim 14, whereinstep c) further includes the steps of determining a position of thecompartment using a positioning system, and retrieving the sample valuefrom a map database based on the position.
 19. The method as claimed inclaim 14, wherein step c) further includes the steps of determining astate of an HVAC system operable to treat the compartment.
 20. Themethod as claimed in claim 14, wherein steps d) and e) further includesthe step of determining a degree of exceedance of the threshold, anddelivering to the element an activation signal proportional to thedegree of exceedance.